Principal Investigator / Author(s): Melack, J M

Contractor: UC Santa Barbara

Contract Number: A3-096-32

Emerald Lake (36E35'N, 118E4O'W) is located in Sequoia National Park at an elevation of 2,780 m in the Sierra Nevada. The lake has one surface outflow and is fed by several inflowing streams that drain a ca. 113 ha catchment; it is 2.85 ha in area and has a maximum depth of 10.5 m. Emerald Lake contains calcium-bicarbonate water with very low acid neutralizing capacity as is typical of high-altitude lakes in the Sierra Nevada. Its current pH (5.6-6.6) is near the low end of the range observed in Sierran lakes. Low values (5.6-6.0) are associated with intense summer rains and snowmelt. During ice-cover and mid-summer the lake is thermally stratified and low dissolved oxygen and elevated concentrations of ammonium, base cations and acid neutralizing capacity (ANC) develop in the deeper water. The major contributors to alkalinity generation in Emerald Lake from the sediments are ammonium production from the breakdown of organic matter (44%) and the exchange of hydrogen ion for calcium in the sediments (34%).

During the initial stages of snowmelt in 1986, sulfate and nitrate concentrations increased while base cations and ANC declined in the subsurface water being influenced by runoff. Inflows varied in pH from 5.7 to 6.5 with minima during snowmelt; inflow ANC ranged from 4 to 45 Feq 1-l with minima during intense summer rain and snowmelt. Aluminum was very low (0.6 to 2 Fm) in the inflowing streams.

Phytoplankton productivity as measured using tracer techniques employing isotopes of nitrogen (15N) and carbon (14C) were low during the ice-free seasons of 1984 and 1985. Carbon uptake rates reached their highest levels in the spring and autumn. Chlorophyll vertical profiles measured throughout 1984, 1985 and 1986 had deep chlorophyll maximum at 7 to 9.5 meters during the summer; mid-winter chlorophyll levels are very low.

Ammonium uptake rate, particulate nitrogen and chlorophyll were used as indices of phytoplankton response to experimental additions of the acids and nutrients associated with acid precipitation to replicated 3,000-4,000 liter bags suspended in Emerald Lake. No significant differences were found among controls and acid treatments without phosphorus. Significant differences are observed among controls and both phosphorus and acid plus phosphorus treatments.

Equations which estimate pH and ANC in high-elevation Sierra Nevada lakes from the species composition of diatom assemblages found in surface sediments were applied to diatom assemblages observed in the upper 20 cm of Emerald Lake sediments. The results indicate that both pH and ANC have varied somewhat since about 1825, but that there is no overall trend in either of these variables.

The zooplankton assemblage in Emerald Lake is dominated by a copepod (Diaptomus signicauda), three cladocerans (Oaphnia rosea, Bosmina longirostris, Holopedium gibberum), and three rotifers (Keratella cochlearis, Polyarthra vulgaris, and Conochilus unicornis). Abundances of all common taxa were highest during the summer and early autumn. Daphnia, Diaptomus and Conochilus were very sensitive to experimental acidic inputs to large bags in the lake with Daphnia and Diaptomus densities declining abruptly as pH was reduced from 5.6 to 5.5. Densities of Keratella and Bosmina, and, in some experiments, Polyarthra, were greater in bags acidified to pH 5.1-5.5 than in non acidified controls probably owing to release from competition with Daphnia. Densities of Keratella, Rosmina and Holopedium were all reduced below pH 5.0.

The zoobenthic assemblages of the Emerald Lake inlet and outlet streams were dominated by chironomid larvae. Simuliid larvae, Baetis nymphs and Rhyacophila larvae were commonly collected in both inflow and outflow streams, particularly on hard substrates, and ephemerellid and leptophlebiid mayflies, nemourid stonefly nymphs, tipulid larvae, water mites, oligochaetes and sphaeriid clams were also commonly collected in the outlet stream. Simuliids, Baetis and Malenka displayed nocturnal increases in their drift rates, mites and chironomids showed different drift patterns, depending on the date or stream sampled, and trichopterans appeared to be aperiodic.

During experimental acid additions to replicate stream channels, Baetis spp. drift in acidified channels was ca. seven times higher than in control channels, and the percentage of drifting baetids that were dead was significantly higher in acidified than control channels. Other taxa showed no significant drift responses. The dominant benthic invertebrates in Emerald Lake were chironomids and sphaeriid clams, and acidification of large bags had no effect on the densities of these taxa.